1. Field of the Invention
The present invention relates generally to an inverter device that performs on-off switching of an input voltage using a switching device so that an excitation current is applied to an excitation winding of a step-up transformer in ON period of the switching device and an output voltage exhibiting a half-wave alternating-current (AC) waveform is delivered from an output winding of the step-up transformer in OFF period of the switching device, a plasma generator apparatus including the inverter device and a control method employed in the inverter device.
2. Description of the Related Art
A switching regulator or an inverter device is used to supply high voltage to a variety of device such as a discharge tube for a large plasma display or a plasma generator apparatus.
Inverter devices that deliver output power of several watts are in common use. However, inverter devices that deliver AC output whose voltage is over ten kilovolts and whose power value is several tens of watts or higher are employed in plasma generator and the like.
A general switching regulator (DC-DC converter) intermittently applies a direct-current (DC) voltage to a primary excitation winding of a voltage-converting transformer by on-off switching the DC voltage using a switching device, and rectifies and smooths an AC current generated in a secondary output winding of the transformer, thereby outputting a DC voltage.
The output voltage is kept at a constant voltage by, for example, pulse-width modulation (PWM) control that controls a ratio (duty ratio) between switching-device-on time and switching-device-off time. The PWM control is performed by detecting the output voltage and producing a feedback voltage based on the detected voltage as disclosed in Japanese Laid-open Patent Application No. 2009-11144, for example.
More specifically, the output voltage is controlled to be constant by increasing ON width of switching pulse when the output voltage drops to thereby compensate for low output power, while reducing the ON width when the output voltage rises to thereby curtail excessively high output power.
As described above, an inverter device intermittently applies a DC voltage to a primary excitation winding of a voltage-converting transformer by on-off switching the DC voltage using a switching device, but delivers an AC voltage generated in a secondary output winding as it is to a load.
Some type of such an inverter device is configured to apply PWM control to the switching device by detecting the output current rather than the output voltage and replacing the detected output current with a voltage corresponding thereto. An example of such an inverter device is disclosed in published Japanese translation of WO2007/060941.
Such a technique disclosed in Japanese Laid-open Patent Application No. 2009-11144 that performs PWM control of a switching pulse for use in on-off control of a switching device by detecting an output voltage is applicable to switching regulators that output DC voltages. Furthermore, because switching regulators have a holding time provided by an electrolytic capacitor or the like in a smoothing circuit on the output side, a problem in control responsiveness does not arise in a switching regulator.
By contrast, because inverter devices deliver AC output, it is difficult to control an inverter device so as to deliver output voltage having a constant crest value (peak voltage value) irrespective of whether the output voltage is a full-wave voltage or a half-wave voltage.
In a condition where an inverter device has a fixed load, and duty ratio, switching frequency, resonant frequency, input voltage, and like circuit conditions that depend on the load are fixed, it can be assumed that change in crest value of output voltage will depend only on an environmental change and a secular change of components. Nevertheless, it is desirable that the crest value of the output voltage is kept approximately constant even if the load varies with temperature, elapsed time, or the like. It is further desirable that a substantially-constant output voltage can be obtained even if the input voltage varies or the load varies dynamically.
Meanwhile, an instantaneous peak output voltage and large difference between the peak and a valley of the voltage make the number of devices necessary for detecting the output voltage large. As a result, a delay in control response time is caused by a parasitic inductance. The effect of the delay becomes more noticeable as the frequency of repetition of the output voltage waveform increases and the crest value drops or rises excessively as the frequency is high.
At worst, the delay can cause resonant frequency drift. If an excitation current of ON period of a next switching cycle flows through the inverter device to which a resonating voltage is being applied, energy of a residual voltage can produce an excessive current, thereby causing resonance to be lost. As a result, electric power exceeding a maximum power rating of the switching device may damage the switching device or transformer saturation may occur.
Thus, use of an inverter device that delivers AC output having a switching frequency of as high as several tens of kilohertz and having a crest value of output voltage as high as over ten kilovolts, which is achieved by making use of voltage resonance, can arise not only a problem of the control responsiveness described above but also problems related to withstand voltages of an output-voltage detector and components, a time period during which the resonance is to be completed, and the like.
However, in such an inverter device as described above, it has been typical that output voltage is not monitored constantly but merely adjusted to an input supply voltage manually set in advance.
Under the circumstances, there is a need for an inverter device that controls its AC output voltage so as to have a constant crest value even when its input voltage varies.
It is an object of the present invention to at least partially solve the problems in the conventional technology.